1. Field of the Invention
The present invention relates generally to a hydraulic power transmission joint for use in the distribution of a vehicle driving force and, more particularly, to a hydraulic power transmission joint rendering its unit lightweight and compact.
2. Description of the Related Arts
Conventional hydraulic power transmission joints are arranged, for example, between a propeller shaft associated with the front differential gear and the rear differential gear, to transmit a torque corresponding to the rotational-speed difference between the input and output shafts. Such hydraulic power transmission joints can include an oil pump type hydraulic transmission joint in which the rotation of a cam having recessed and raised portions thrust the plunger or the vane to displace oil. In the oil pump type hydraulic transmission joint, the oil must be discharged when the cam is in its descending stoke but sucked when it is in its ascending stroke. One way valves are therefore needed in order to prevent a reverse flow of oil from the high pressure chamber upon the intake as well as an oil leakage into the low pressure chamber upon the discharge.
One example of such a hydraulic power transmission joint equipped with one way valves is described in Japan Patent No. 98-164628. FIG. 1 is a sectional view of the joint disclosed in Japan Patent No. 98-164628, and FIGS. 2A and 2B are explanatory diagrams of operations of FIG. 1 joint. In FIGS. 1, 2A and 2B, a rotor 101 has a plurality of grooves formed in its outer peripheral portion. Each groove receives a vane 102 slidably inserted thereinto. To effect the function as a hydraulic pump, the relative rotations between a cam ring 103 and the rotor 101 cause a generation of hydraulic pressure within pump chambers 104, 105 and 106. The discharge ports of the pump chambers 104, 105 and 106 are blocked so that the rotor 101 and cam ring 103 can rotate jointly like one rigid body by the hydrostatic pressure. Intake/discharge ports 111, 112 and 113 serve as intake ports or discharge ports depending on the direction of rotations of the vanes 102 and communicate mutually with a first oil passage 114. Similarly, intake/discharge ports 107, 108 and 109 serve as intake ports or discharge ports by the action of the vanes 102 and communicate mutually with a second oil passage 110. The intake/discharge ports 111, 112 and 113 resulting in the intake ports or the discharge port at one time communicate with one another through the first oil passage 114. The first oil passage 114 connects with the second oil passage 110 by way of check valves 120 and 121 acting as one way valves. When the vanes 102 relatively rotate counterclockwise as in FIG. 2A for example, the check valve 120 is closed to separate the high pressure side from the low pressure side while simultaneously the check valve 121 is opened allowing a communication with the high pressure side. As a result, the discharge ports of the pump chambers 104, 105 and 105 are blocked to generate a hydrostatic pressure, the thus confined oil causing the rotor 101 and the cam ring 103 to rotate jointly like one rigid body, for torque transmission. On the contrary, when the vanes 102 relatively rotate clockwise, the check valve 121 is closed to separate the high pressure side from the low pressure side, while simultaneously the check valve 120 is opened to allow a communication with the high pressure side. In consequence, the discharge ports of the pump chambers 104, 105 and 106 are blocked to generate a hydrostatic pressure, the thus confined oil causing the rotor 101 and the cam ring 103 to rotate jointly like one rigid body, for torque transmission. The first oil passage 114 and the second oil passage 110 open to an oil reservoir 115 by way of check valves 116 and 117 serving as one way valves permitting only the flow from the oil reservoir 115. Such a hydraulic pressure circuit allows an action of discharge pressure proportional to the rotational speed at all times, irrespective of the direction of the relative rotations.
A cover 118 of FIG. 1 includes a passage 119 for allowing an action of high pressure for thrusting up the vanes from annular recessed grooves provided in the rotor side surfaces, the passage 119 communicating with the intake/discharge ports 111, 112, 113, 107, 108 and 109 of FIGS. 2A and 2B. The discharged high pressure oil is allowed to circulate through orifices 122 and 123 communicating with each other of the first and second oil passages 114 and 110. Check vales 120 and 121 are further incorporated for the constant action of high pressure on the bottoms of the vanes. The two check valves 120 and 121 serve to prevent any reverse flow of oil from the high pressure side through the intake passage as well as any oil leakage to the low pressure side through the discharge passage.
However, such a conventional hydraulic power transmission joint needs four one way valves which are incorporated in the both side cover portions making up the hydraulic chamber of the vane pump, with the result that a wider accommodation space is required and thus the cost itself also increases. Furthermore, the oil passage tends to become longer since the discharge ports and the intake ports of a plurality of hydraulic chambers are allowed to communicate with two check valves in common. In particular, an elongated intake path may bring about a defective intake due to the oil line resistance, and if the oil viscosity is high at the low temperature, the vanes or the plungers may fail to follow up, result in an occurrence of foreign noises.
Such a one way valve problem will apply similarly to the case of use of the one way valve in the hydraulic power transmission joint having a structure where the rotation of the cam thrusts the plungers in the axial direction to displace the oil, as in U.S. Pat. Nos. 5,706,658 and 5,983,635.